Gypsum board wastes amount to about 1,500,000 tons each year in Japan. About 500,000 tons out of this is end materials and remainder materials generated from the production of gypsum boards, the construction of new buildings and the interior finish work of buildings, and gypsum board manufacturers recycle these materials. The remaining about 1,000,000 tons of the gypsum board wastes is wastes discharged from the remodeling and demolition of buildings. The wastes contain board body paper and metal pieces such as screws, nails and wires. Therefore, to recycle the wastes, foreign matter such as board body paper and metal pieces must be removed. However, as the process of removing the above foreign matter is too troublesome, the wastes are landfilled without being recycled.
When gypsum is contacted to water, it may produce hydrogen sulfide. Therefore, the gypsum board wastes must be placed in a controlled landfill, and its disposal cost poses a problem to be solved. In addition, the amount of gypsum board wastes is increasing each year, and an efficient pre-treatment method for recycling gypsum board wastes is desired to cope with a shortage of landfill sites and reduce an environmental burden.
UP till now, some methods for removing metal pieces from gypsum board wastes have been proposed. The methods include one in which gypsum board wastes are crushed and the obtained crushed pieces are sieved to remove metal pieces on the sieve, and one in which a magnetic separator is used to magnetically adsorb metal pieces from crushed pieces of gypsum board wastes. However, according to these methods, when gypsum board wastes are to be crushed, a crushing machine may be broken by metal pieces contained in the wastes.
Then, methods for removing metal pieces from gypsum board wastes before crushing are under study. The methods include one in which a pendant magnetic separator is installed above the path for carrying gypsum board wastes to a crushing machine by means of an automatic carrier device and one in which a magnet pulley is provided on part of the carrier surface of an automatic carrier device. However, metal pieces on the lower side of each gypsum board waste cannot be removed in the former method and metal pieces on the upper side of the gypsum board waste cannot be removed in the latter method. That is, in these methods, the metal pieces on the upper or lower side of the gypsum board waste cannot be removed. To remove all metal pieces on the upper side and the lower side of the gypsum board waste by means of a magnetic separator, magnets of two systems must be installed by combining these methods, which is very uneconomical. Even when magnets of the two systems are installed by combining the above methods, metal pieces sandwiched between two gypsum boards placed one upon the other cannot be removed.